I am going to say its not the relief valve. At idle, the motor is not generating enough RPMs to push enough oil to create an 80+PSI back pressure. Remember... pumps create FLOW. restrictions create PRESSURE.
At idle the relieve valve would never lift even in a properly working motor, as the normal oil pressure, at idle, is less than 10PSI (some trucks simply say 'any' pressure is OK.
Since this motor is new to you, and has had extensive internal work "i pulled the rod caps and main caps all looked good someone had just did alot of work on the engine because it had new rods, pistons,and oil pump
Sounds to me like something was wrong (you don't replace all that stuff for kicks and giggles) and is still wrong.
Somethings off the top of my head:
Are the bearing in right (oil holes not covered)?
Are the cam bearings in right (oil holes not covered)?
Is any oil making it to the lifters? A late model 261 is going to be fed oil via the lifter galley.. DO YOU HAVE THE RIGHT LIFTERS IN?????
So basically you pump is working fine, the relief valve would never lift at idle on a working motor... that leaves clogged/blocked oil galleys.
When you find out what's blocked, let us know.
